GEOCHEMICAL EVIDENCE FOR ENHANCED PRESERVATION OF ORGANIC-MATTER IN THE OXYGEN MINIMUM ZONE OF THE CONTINENTAL-MARGIN OF NORTHERN CALIFORNIA DURING THE LATE PLEISTOCENE

The present upper water mass of the northeastern Pacific Ocean off Cal ifornia has a well-developed oxygen minimum zone between 600 and 1200 m wherein concentrations of dissolved oxygen are less than 0.5 mL/L. E ven at such low concentrations of dissolved oxygen, benthic burrowing organisms are abundant enough to thoroughly bioturbate the surface and nearsurface sediments. These macro organisms, together with micro org anisms, also consume large quantities of organic carbon produced by la rge seasonal stocks of plankton in die overlying surface waters, which are supported by high concentrations of nutrients within the Californ ia Current upwelling system. In contrast to modem conditions of biotur bation, laminated sediments are preserved in upper Pleistocene section s of cores collected on the continental slope at water depths within t he present oxygen minimum zone from at least as far north as the Calif ornia-Oregon border and as far south as Point Conception. Comparison o f sediment components in the laminae with those delivered to sediment traps as pelagic marine ''snow'' demonstrates that the dark-light lami nation couplets are indeed annual (varves). These upper Pleistocene va rved sediments contain more abundant lipid-rich 'sapropelic'' (type II ) organic matter than the overlying bioturbated, oxidized Holocene sed iments. The baseline of stable carbon isotopic composition of the orga nic matter in these slope cores does not change with time, indicating that the higher concentrations of type II organic matter in the varved sediments represent better preservation of organic matter rather than any change in the source of organic matter.